Infrared spectroscopy has been utilized for identifying gaseous reagents within a confined region such as a test chamber for comparing the spectrum of the gaseous reagents with the spectrum of a known gas in a reference chamber. Typically, a laser is operated to produce various spectral lines within the region of the expected absorption spectra of the unknown gaseous reagents such that the amplitudes of the observed spectrum can be correlated or compared with the spectra of known chemicals to identify the unknown reagent. The laser spectrometers may be configured to provide for a reference signal to aid in the reception of the optical signal radiation from the test chamber to permit a heterodyning of a received frequency with the reference frequency to produce a beat frequency signal which is readily processed electronically for identifying the frequency and amplitude of the beat frequency signal. One such arrangement is disclosed in the U.S. Pat. No. 3,856,406 which issued in the name of Noble et al. on Dec. 24, 1974.
A problem arises in the obtaining of identifying spectral signatures of chemicals at a long distance, through the air, such as airborne gaseous reagents a number of miles away from the spectrometer. Difficulties exist because of the ever present dust particles or rain drops suspended within the air along the optical path via which the optical radiation must propagate between the spectrometer and the unknown chemicals. The dust or water may attenuate the received signal to the point where an accurate identification of the unknown chemicals is impossible in the absence of a very high gain receiver for the received signal at the spectrometer. An amplifying medium of suitable gain for the aforementioned situation is the medium of an oscillating laser such as that disclosed in the U.S. Pat. No. 3,950,100 which issued in the name of Keene et al, on Apr. 13, 1976 which teaches that a power gain, even higher than the gain of the laser at the frequency of oscillation can be obtained at a frequency offset therefrom by approximately 250 kilohertz (kHz). The gain at the frequency of oscillation is reduced from that of the offset frequency, in the case of a carbon dioxide laser, by a carbon dioxide depletion region, the depletion region being absent at the 250 kHz frequency offset. The gain provided by such a lasing medium is substantially higher than that which can be obtained by a lasing medium which has not been excited to the point of oscillation. However, the restriction to a specific frequency offset, required for the high gain amplification is not compatable with the teaching of Nobel wherein differing frequency offsets are utilized as an indication of a specific spectral line.